1. Field of the Invention
This invention relates to group II-VI thin film transistors, a method of making same and a monolithic device containing a detector array as well as transistors coupled thereto.
2. Brief Description of the Prior Art
Infrared focal plane arrays presently fabricated on group II-VI compounds, particularly HgCdTe, require electrical signals to be input and output from the focal plane array. Output signals require some form of signal processing (i.e. amplification, buffering, etc.). Input signals provide clocking or power supply to the various elements of the array.
While such infrared focal plane arrays are presently realized in group II-VI compounds, functions such as signal processing and clocking are performed almost exclusively in silicon. While incorporation of circuits into the group II-VI material itself along with the focal plane array has many advantages, such as increased array responsivity, fewer leads to the focal plane array, etc., this concept to date has been realized to only a limited extent.
Though many types of transistor structures have been demonstrated in group II-VI compound materials, they are not compatible with all (or necessarily any) detector structure and, in addition, many have limitations resulting from the small band gap of the material in which they are fabricated.
One approach that has been taken to alleviate the above described problem of the prior art has been to fabricate appropriate circuitry on silicon and bond these circuits to the HgCdTe focal plane array. In bonding the silicon to the HgCdTe, a significant stray capacitance is introduced, substantially diminishing the output signal from the focal plane array and necessitating additional amplifier circuits in the silicon. To minimize the stray capacitance, the silicon circuits are located in close proximity to the HgCdTe array, usually on the focal plane itself. This is an additional weakness since it increases power dissipation in the focal plane array.
Another approach is to incorporate some signal processing functions into the HgCdTe itself using MISFET circuitry. This significantly reduces stray capacitance problems, but has several weaknesses as well. Circuitry of any complexity will require the connection of the source or drain of one FET to the gate of another. Since the source and drain regions of MISFETs are formed by ion implanted diodes, the operating voltages of gates are constrained to the range between the forward turn-on voltage of the diode and its reverse breakdown voltage. Since the reverse breakdown is less than one volt on long wavelength HgCdTe, this represents a severe limitation. In many cases the fixed charge density at the HgCdTe insulator/interface does not permit operation in this range.
In addition, MISFETs can only be fabricated on p-type HgCdTe since a device compatible method of producing p-type regions on n-type HgCdTe has not been demonstrated.
Several transistor types have been demonstrated in the literature on group II-VI materials and all but one rely upon HgCdTe for the conduction channel. These various transistor architectures invariably impose constraints on film thickness, conductivity type and band gap that are at odds with detector requirements.
All transistors require CdTe or mid-to-wide band gap HgCdTe for optimum performance. This is at odds with long wavelength detection, requiring narrow band gap HgCdTe. As referenced in an article of A. Kolodny et al., IEEE Trans. Electron Devices, ED-27, 591 (1980), MISFETs are practical only on p-type material. For many detectors, n-type HgCdTe is the material of choice. As referenced in an article of G. R. Chapman et al., U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride, Orlando, Fla. (1988), JFETs and as referenced in an article of D. L. Dreifus et al., Jour. Vac. Sci., U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride, San Diego, Calif. (1989), thin film transistors require thin films on insulating substrates. Yet two-color detectors require multiple conductive films of HgCdTe. For MISFETs, JFETs and thin film transistors to be practical for widespread use, many additional film layers must be grown, solely for the transistor-circuits, adding to the complexity and risk of the material growth and device fabrication process.
As referenced in an article of T. Ashley et al., Electron. Lett., 22,611 (1986), bipolar junction transistors have also been demonstrated in HgCdTe, but not in a form that is of practical value to the fabrication of circuits.
As referenced in an article of D. L. Dreifus et al., Applied Physics Letters, 51, 931 (1987), MESFETS have been demonstrated in conductive CdTe. This is the only transistor type thus demonstrated in a group II-VI compound that is compatible with any detector type and not coupled to properties of HgCdTe layers.